Proportioning solenoid

ABSTRACT

A solenoid and solenoid-actuated mechanism which is operated under conditions whereby the armature of the solenoid is controllably and variably movable by selected increments by the imposition of such current levels, with respect to armature mass and load forces acting thereon, so as to obtain high armature saturation and by the provision of a solenoid design that permits high magnetic leakage paths so that the armature displacement is a function of input control current which may in several embodiments include an interrupter or switching circuit. Hysteresis is beneficially influenced by the reduction of frictional resistance parameters to obtain high performance characteristics.

D United States Patent 1 [111 3,725,747 Cowan [451 Apr. 3, 1973 [54] PROPORTIONING SOLENOID 3,424,951 1/1969 Barker ..317/123 75 t A It! A. T f. Inven or rno Cowan, arzana, Cali Primary Examiner Ll T. Hix [73] Assignee: DeLaval Turbine Inc., Princeton, Atm Th P, M h et 1 22 Filed: Jan. 17,1972 [57] ABSTRACT A solenoid and solenoid-actuated mechanism which is 21 A LN 21 40 l 1 PP operated under conditions whereby the armature of Related [1,5 A li tion mm; the solenoid is controllably and variably movable by 63 I selected increments by the imposition of such current lcgpltmubatlojn-m-pa t 0f 4 July levels, with respect to armature mass and load forces an one acting thereon, so as to obtain high armature saturation and by the provision of a solenoid design that perggi] US. Cl mits high magnetic leakage paths so that the armature M is h 317 123 displacement is a function of input control current le 0 earc which may in several embodiments include an inter rupter or switching circuit. Hysteresis is beneficially [56] References cued influenced by the reduction of frictional resistance UNITED STATES PATENTS parameters to obtain high performance characeristics. 2,989,666 6/1961 Brenner et al ..3l7/123 3,378,732 4/1968 Dietz et al. ..317/123 26 Claims, 12 Drawing Figures PATENTEBAPRIB ms 3,725,747

SHEET -1 [1F 2 FIG.

FIG. 2 F163;

IN VE N TOR. ARNOLD A. COWAN B /l/IA HONEY, HORNBA KER AND SCH/CK ATTORNE Y8 P ATEIETEUAFM ms SHEET 2 BF 2 PROPORTIONING SOLENOID CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of Cowan application, Ser. No. 164,527, filed July 21, 1971 and now abandoned.

BACKGROUND OF THE INVENTION Conventional solenoid valves and associated mechanisms, as for example, valves used to regulate fluid flow, have not been of the type that are easily controlled merely by varying input voltage. By this is meant that in the past, such mechanisms have not been of the type that would permit operation thereof on a reliable basis wherein the armature stroke is varied to achieve a desired end effect merely by varying the input current to the solenoid. These valves have been of the on-off variety with the armature being positioned at the extreme end of its travel. That is, total armature displacement is dependent on the solenoid being energized or de-energized. Additionally, inherent hysteresis influenced by magnetic and mechanical resistance factors has been difficult to control for selected operating performance of these devices.

With the herein disclosed invention an electromechanical control device is disclosed which can fulfill a variety of needs where low cost proportioning devices may be useful, as for example, to meter or control fluid flows or pressures. This essentially is accomplished by designing the solenoid of the mechanism so that the armature displacement is a function of an input control current and by keeping hysteresis within certain limits by substantially reducing frictional drag or mechanical resistance of the dynamic components of the devices.

In basic configuration, the solenoid or solenoid component of the devices disclosed herein are similar to conventional solenoid design. The unique performance is achieved through a specially configured armature, control of the air gap, the provision of pole shading and in some instances, the provision of a non-linear rate armature spring. In some embodiments an interrupter circuit, utilizing the solenoid coil, is also used. Additionally, friction or mechanical resistance is substantially reduced by several expedients for obtaining devices of high performance characteristics.

For example, where fluid flows are to be controlled an appropriate valve seat arrangement may be mounted to the solenoid so that the armature can be made to actuate or to serve as a poppet or spool so that the mechanism can perform as a variable orifice valve.

This valve in the normally closed position will have the poppet covering the valve seat when the solenoid is de-energized. When the solenoid is energized, the valve orifice is uncovered to provide an increasing orifice area as the poppet moves away from the valve seat. Since the armature displacement is a function of the input current, the orifice area also becomes a function of the input current, i.e., the change in orifice area is a function of a change in current. Through the greatest part of its range this relationship is practically linear and constant using the principles of this invention.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of this invention to provide a method of operating solenoids.

It is another object of this invention to provide a method for operating solenoids and solenoid actuated mechanisms under comparatively linear armature response conditions.

It is still another object of this invention to provide a solenoid that is of unique design, especially adaptable for association with solenoid-actuated mechanisms, wherein the armature movement of the armature or mechanism is simply controlled by input current to the solenoid, in one embodiment using a switching or interrupter circuit and wherein frictional drag or mechanical resistance between armature are core is substantially reduced by expedient means comprising the armature, core or both.

It is still a more specific object of the invention to provide a unique solenoid and method of operating same so that operation of the device is carried out at relatively high current levels relative to armature mass and imposed force loads and under specific conditions whereby substantially linear response of the armature correlative to current input is obtained.

It is still another and more specific object of the invention to provide a solenoid-actuated valve mechanism wherein the armature of the solenoid is of specific configuration and the input current levels are sufficiently high to obtain linear responsiveness of the armature whereby valving actions are performed to control fluid flow or pressures.

These and other objects of the invention will become apparent as the description proceeds herein when taken in conjunction with the drawing.

Basically, in an exemplary embodiment, the invention pertains to a uniquely configured solenoid having shading of the armature and employing high magnetic leakage paths and a method of operating such solenoid and solenoid actuated mechanisms under reduced mechanical resistance conditions. Generally, the method comprises operation under comparatively linear armature response conditions comprising the steps of energizing the solenoid at high current levels relative to armature mass and imposed loads and maintaining said levels within selected limits and providing high magnetic leakage paths for said solenoid and selectively varying said current within said selected limits to obtain substantially linear response of said armature correlative to said current and by limiting hysteresis affect by reducing mechanical resistance. In some instances an interrupter circuit, dependent upon and utilizing the solenoid coil, is used to impose exciting armature current. Alternatively, a switching circuit may be used instead of the interrupter circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view partially broken away of a conventional solenoid modified in accordance with this invention;

FIG. 2 is a view taken along the line 2-2 of FIG. 1;

FIG. 3 is a graph relating to the hereindisclosed invention;

FIG. 4 is a graphical representation of the salient import of the invention;

FIG. 5 is a schematic diagram depicting an interrupter circuit for use with the solenoid shown in FIG. 1;

FIG. 6 is a schematic diagram illustrating a switching circuit for use with the solenoids of this invention;

FIG. 7 is an elevational view, partially in cross-section of another embodiment of the invention;

FIG. 8 is a view taken along the line 8-8 of FIG, 7;

FIG. 9 is a fragmented, cross-sectional view of another embodiment of the solenoid devices of this invention;

FIG. 10 is a view taken along the line 10-10 of FIG. 9;

FIG. 11 is a fragmented, cross-sectional view of still another embodiment of the solenoid devices of this invention; and

FIG. 12 is a view taken along the line l2--l2 of FIG. 1 1.

DESCRIPTION OF THE BEST EMBODIMENTS CONTEMPLATED Referring to the figures of the drawing wherein like numerals of reference designate like elements throughout, the solenoid mechanism 2 is generally as conventionally found in existing solenoids. Basically, as in conventional construction, an outer casing 4 forms a central bore 6 within which the conventional core 8 surrounded by coil 10 is disposed. The casing 4 in some instances will be of a configuration to accommodate interrupter or switching circuitry as will become apparent.

It will be noted that the core 8 is provided with a conical end 12. Through core 8 is disposed adjusting screw 14, one end of which retains the armature return spring 16. Adjusting screw 14 permits adjustment of the load and rate of spring 16. The armature 18 has an end 20 congruently shaped to the end 12 of core 8 to provide conical air gap 22 (g) as is conventional in the solenoid art.

Armature 18 is provided with a central recess 21 to receive return spring 16 in the manner illustrated. In this particular instance, return spring 16 is of a type that has a selected K value or characteristic in order to facilitate operation of the solenoid or solenoid associated device in the mode contemplated by this invention. More about this will become apparent as the commentary proceeds.

Armature 18, in this particular instance, in order to provide for as great a flux leakage and saturation loss as possible, is shaded as at 24, the shading being produced by the provision of groove 26 in the armature body. In this particular instance, the main portion of the armature 18 is provided with an orifice or slot 28 which in the embodiment depicted, is intended to operate as a linearly responsive valve. Thus, in a fluid environment, the groove 28 permits pressure equalization from the surrounding ambient environment and the bore or path through which the armature l8 linearly moves.

In the operation of the hereindisclosed proportionning solenoid, it will be noted that at any given point in the displacement of the armature 18, there will I be a balance between the tractive force of the electromagnet formed by the coaction of the stationary core 8 and the coil 10 upon energization and the resistive force of the v armature or plunger 18 and the return spring 16, and an external load, if any.

A change in current will produce a change in the magnetic force and one of the flux paths being shown in dash lines in FIG. 1, and will tend to displace the plunger or armature 18. This movement will compress or extend the spring 16 until a new point of equilibrium is reached.

In these states of equilibrium, ZF O. Then F F, F 0, where F Magnetic Force, F, Spring Force,

(4) B r cos 22.9

2 (5) NI=W+KNI where,

B=flux density r=radius of armature a=c0ne angle armature u=permanability of air gap g=air gap or stroke N =number of turns in coil Ic=magnetic losses (usually .2)

f I=current in amperes cos 0:9

and by combining (4) and (6) is obtained.

In any given solenoid configuration, r, N, u k and a are constant; the only variables are I and g. Thus, equation (7) may be written:

where C product of all constants as shown in equation 7. If the current I were held constant, then it could be included among the constant factors and equation (8) could be written:

Equation (9) describes a family of force-stroke curves following the Inverse Square Law. This is graphically illustrated in FIG. 3 for various values of current (I).

Since the principal area of interest is in a linear relationship between current, stroke and force, FIG. 3 illustrates a family of typical force-stroke curves for linearly incremental currents.

It should be noted that a line which intersects curve 1 at g and curve I at g etc., will be a force-stroke curve which derives from a relationship between current and stroke. The line AB then shows the magnetic force (F,,,) at a given stroke and current.

It is apparent that the slope of this line represents the spring rate characteristics necessary to achieve equilibrium in operation. By inspection this is seen to be non-linear (since the family of force-stroke curves are non-linear). It is infeasible to achieve this nonlinear characteristic with practical spring design.

However, a simple compression spring may be provided to produce a near-linear resistive force. Should the design limitations prevent the achievement of nearlinearity, the curve can be divided into two sections, each of which has a different slope but near-linear within that section. Contemplated to accomplish this is the use of two concentric springs to achieve a dual rate resistive force which is based on the average linearity. Other modes are obviously contemplated, e.g. an elastomeric cylinder configured to produce a variable rate response.

It is characteristic of tractive-type electromagnets that the theoretical force-stroke curves follow the Inverse Square Law. In actual practice, however, the curves approach the ideal form at lower currents and tend to flatten out as higher currents are applied. This may be explained as the effects of iron saturation and magnetic leakage. This phenomenon is utilized as one of the factors in the herein disclosed invention to produce the linearity of the armature or plunger displacement with respect to the input current.

In practice, saturation can be achieved by using higher currents relative to armature mass and load factors thereon, while magnetic leakage can be increased by providing greater leakage paths as by the provision of specific air gap configurations appropriately located as illustrated herein. Generally, current ranges within about 50 1000 miliamps will be used.

According to BC. Roters, the precise mathematical calculation of the permeance of flux paths through air, or the more general problem of the determination of solenoidal fields, except in a few special cases of the more simple configurations, is a practical impossibility. Consequently, design formulas for the herein disclosed invention have been developed through the modification of conventional design formulas and by making simplifying assumptions and by establishing empirical values and working ratios covering:

( l magnetic leakage factors (K),

(2) relationship between air gap configuration vs. ar-

mature sizes, and

(3) stroke vs. air gap for cone angle of the armature.

For example, additional leakage may be produced by necking down the back end of the plunger. Typically the movement of the plunger is from right to left as viewed in FIG. 1, in a manner that there is always an air gap throughout the whole stroke of the armature. The combined effect of the increased current and magnetic leakage is shown graphically in FIG. 4, a specific showing analogous to FIG. 3.

Line C represents. the low current curve, line D represents the high current curve and line E represents the inverse force curve.

Note how the curve D flattens and tends toward linearity as the stroke increases. It can also be shown graphically that a decrease in the cone angle for a given coil, current, and armature diameter will also decrease the slope of the force-stroke curve.

The effect of increased magnetic leakage for a given current and air gap is to decrease the force. The attainment of a specified greater force will, therefore, require a greater current; which in turn has the further effect of flattening the curve D.

The overall effect of these various control factors is to produce a force-stroke curve that is near-linear in the region of the increased air gap; that is, the stroke range designated X in FIG. 4 and within which the solenoids and method of operation of this invention are carried out.

In specific instances, non-linear flow of an associated valve mechanism may be produced by developing an appropriate configuration of the valve element and its seat so that the linear displacement of the valve element will produce a non-linear change in orifice area. In other instances of end use application, a pressure balanced valve mechanism may be provided to eliminate the imposition of a variable external load on the armature return spring so that the change in orifice area and current are unaffected through the pressure range.

Referring to FIG. 5, an interrupter circuit is illustrated which in some embodiments is used in conjunction with the solenoid shown in FIG. 1 and which circuit is adapted to be housed within the casing 4 and use the coil 10.

The circuit, as indicated includes a coil 34 which may or may not be the coil of the solenoid but, which is in the preferred embodiment, diode 36, resistors 38 and 40 wired as shown and transducers 42 and 44 with capacitor 46.

Typical valves for specific applications contemplated would be:

coil 34 500 resistor 38 l5.Q, IW resistor 40 3300, H4 W capacitor 46 2/50 MFD, 10v.

The conventional circuit of FIG. 5 is used to provide a dither in the armature 18 of solenoid 2 where low hysteresis (minimum lag between change in input current and the displacement of the armature) is important.

During operation of solenoid 2, when the current is momentarily interrupted, the armature return spring 16 urges the armature 18 towards its closing position. As the armature begins to move, the current flow is resumed. This in turn generates the magnetic field which then returns the armature to its original position. The current is again interrupted and the cycle is repeated.

In the schematically illustrated circuit, the cycling is repeated 50 to 60 times per second to provide the necessary dither, the purpose of which is to minimize the response time between a change in the signal current to the armature movement.

The foregoing interrupter circuit may be used where the overall circuit does not permit a current control member and thus may be considered voltage sensitive. However, of more general application is the switching circuitry of FIG. 6. I

Referring to FIG. 6, it will be seen that the switching circuitry 50 allows for the same general operation, previously described, for the solenoids of this invention. However, in lieu of the dither or interruption effected by the interrupter circuitry, circuitry 50 permits switching or by-passing to achieve the same end result while permitting the use of continuous current flow and thus may be considered current sensitive.

Preferably the circuitry 50 includes the solenoid coil 52, resistors 54, 56 and 58; diode 60; transducers 62 and 64 and capacitor 66 wired as shown.

Typical values for the switching circuit 50 would be in the range of the following:

Resistor 54 20 Resistor 56 3 K Resistor 58 l MEG Capacitor 66 0.l MFD As briefly alluded to hereinbefore, there are two important performance characteristics for the proportioning or solenoid actuating devices of this invention. The linearity response of the devices is, of course, important as already described, and the second important performance characteristic is that dependent upon hysteresis.

The parameter hysteresis is affected by essentially two factors. Firstly, magnetic hysteresis is an inherent metal property of the magnet utilized in the construction of the solenoid devices of this invention, but there is little that can be controlled or changed with respect to this factor except to use metals of high permeability. The second factor involved in the overall characteristic is that of mechanical or frictional resistance caused by the frictional forces acting upon the armature of the solenoid devices of this invention. By reducing or minimizing frictional forces or mechanical resistance between the interface of the metal armature and spool or core of the solenoid structure making up the coil, a device which is especially useful for metering valve applications having high performance response is obtainable.

One means of selectively controlling mechanical resistance or friction in order to obtain the beneficial effects thereof is by the use of low friction materials as will be more fully developed, and/or by the use of low micro-finishes on the interface surfaces of bore and armature. Because the formation of micro-finishes is necessarily controlled by economic factors, a more feasible approach is through the use of low friction materials as will now be described.

Referring to FlGS. 7 and 8, a device 72 of this invention especially suitable for metering valve application is shown wherein mechanical hysteresis is substantially reduced so as to enable the device 72 to be more responsive and more readily controlled for its intended function. Herein, the solenoid device 72 is adapted to be operable within a fluid environment such that the armature 74 disposed within the casing 76 operates within a fluid medium to exert a metering or valving effect by the means of a valve element (not shown) secured to the end 78 of armature 74. It will be noted that in this structure, in order to preserve fluid tightness and thus operability, that O-ring seals such as 80 are utilized for their well known purposes. Adjustment screw 82 as well as coil 84 are as described for the earlier device 2. It will be noted thatthe armature 74 is of slightly different configuration because of the shading involved and utilizes a conventional coil spring 86 again as for the earlier described embodiment. It will be noted, however, that the device 72 differs essentially in the formation of the bore or core 88 within which the armature 74 is reciprocally disposed. Thus, where heretofore the bore 88 was comprised of a metallic cylinder, the bore is now formed by sleeve 90 constituting a portion of the spool structure 92 upon which the coil 84 is wound. Preferably, at least the sleeve 90 is of a synthetic self-lubricating type of material having a low coefficient of friction between it and the surfaces of the armature 74 with which it makes contact. A satisfactory type of material has been found to be teflon, but obviously other types of synthetic or natural materials will suffice so long as they meet the criteria of being inert within the environments which they are used, offer low mechanical resistance or friction to the movement of the armature 74 therein, and fulfill the necessary electrical parameters intrinsically necessary in devices of the solenoid type. To obtain greater performance, the armature 74, and specifically the exterior surfaces thereof may have a micro finish or may be nickel plated, for example, as to further lower the frictional resistance between it and the sleeve 90.

As earlier stated, the bore need not necessarily be formed by a sleeve of synthetic or natural material of low frictional characteristic, but may be of metal having a high performance micro finish so as to diminish the frictional drag or resistance to movement of the armature 74 therein. Alternately as will now be described with respect to the remaining figures, the micro finishes may be dispensed with and other means of Obtaining high performance of the devices of this invention resorted to.

For example, referring to FIGS. 9 and 10, the device 102 illustrated therein utilizes a metallic member or spool 104 upon which the coil is wound, thereby forming a metallic bore 106 within which the armature 108 is disposed. in order to reduce frictional drag and minimize a hysteresis factor caused by mechanical resistance, the armature 108 is modified so as to receive low coefficient friction buttons 110 circumferentially placed about the surface of the armature 108 at the fore and aft ends thereof. Thus, the reciprocal movement within bore 106 is obtained by the buttons 110 engaging the sleeve 104. Suitable material for the friction reducing buttons 110 may again be any one of the materials mentioned for the member of the device 72 heretofore described. Obviously, the armature 108 with its friction reducing buttons 1 10 may be utilized in the device 72, and more specifically with a sleeve of low friction material so that frictional resistance or drag is even further reduced.

Another means of further reducing frictional resistance is through the use of piston-type rings of low friction material. Referring to FIGS. 11 and 12, the device 112 is shown as utilizing a metallic spool 114 (obviously it may be of low frictional plastic material such as teflon) forming a bore 116 within which the armature 118 is disposed for reciprocal movement. Instead of the friction reducing buttons at in the FIG. 9 embodiment, the mechanical resistance of armature 118 with respect to the bore 116 is reduced by the utilization of piston ring-type members 120, again of a low coefficient of friction synthetic material meeting some of the criteria as briefly described hereinbefore and others of which will be obvious to those of skill in the art. Obviously, the bore 116 may be of the same material as friction reducing means 120 or may be formed by a metallic member having a high micro finish so as to lower frictional drag between the armature 118 and the bore within which it moves. Other armature configurations, modes of reducing frictional drag and suitable materials will suggest themselves to those of ordinary skill in the art, all of which will not depart from the essence of the invention and will merely involve the exercise of ordinary skill.

Thus, there has been disclosed and described a unique solenoid configuration and a method of operating same which will have a myriad of end uses and applications. For example, the proportioning solenoid disclosed herein may serve as a force motor which may be used to actuate a spool and sleeve type valve mechanism. It may also be used for variable orifice valves, metering valves, by-pass valves and flow control valves, where, of course, the plunger or armature of the solenoid is appropriately associated with such valving mechanisms.

These and other applications and end uses as well as various modifications and changes in the specific structure illustrated will make themselves known to those of ordinary skill in the art, all of which are intended to be covered by the appended claims.

lclaim:

1. The method of operating solenoids and solenoidactuated mechanism under comparatively linear armature response conditions comprising:

a. energizing the solenoid at relatively high current levels with respect to the mass of the armature of the solenoid and load forces thereon and maintaining said levels within selected limits;

b. providing high magnetic leakage paths for said solenoid; and

c. selectively varying said current within said selected limits to obtain substantially linear response of said armature correlative to said current.

2. The method in accordance with claim 1 wherein said current levels are within the near-linear areas of the force versus stroke solenoid curves.

3. The method in accordance with claim 1 wherein said high magnetic leakage paths are obtained through the provision of air gaps between said armature and the core of said solenoid.

4. The method in accordance with claim 3 wherein additional high magnetic leakage paths are obtained by shading the diameter of said armature.

5. The method in accordance with claim 4 which additionally includes varying the solenoid return spring characteristic in order to obtain said linear response.

6. The method in accordance with claim 5 wherein the current supply to the coil of said solenoid is discontinued about 50 to 60 times per second.

7. The method in accordance with claim 6 wherein the armature is operated under dither conditions, the current impulse cycle urging said armature toward the open position and said solenoid return spring urging said armature toward the closing position.

8. The method in accordance with claim 4 which additionally includes obtaining said linear armature response under low mechanical resistance conditions.

9. The method in accordance with claim 8 wherein the coefficient of friction of the materials comprising said armature and said core are maintained at a minimum.

10. The method in accordance with claim 8 wherein said low mechanical resistance conditions are obtained by providing point contacts between armature and core of low frictional drag characteristics.

11. In a solenoid comprising a core; shiftable armature and actuating coil, the improvement which comprises means to establish high saturation losses during operation of said solenoid in order to obtain substantially linear response of said armature correlative to increase in current inputs to said coil.

. The improvement in accordance with claim 11 movement of said armature upon actuation of said sole-- noid.

14. The improvement in accordance with claim 13 wherein said shading to said armature comprises an annular groove formed in the end of said armature furthest from said core.

15. The improvement in accordance with claim 14 which additionally includes the formation of a conical air gap between the core and armature end opposite said groove.

16. The improvement in accordance with claim 15 which additionally includes circuitry to periodically energize said solenoid.

17. The improvement in accordance with claim 16 wherein said circuitry is located within the housing of said solenoid.

' 18. The improvement in accordance with claim 17 wherein said circuitry is an interrupter circuit utilizing said actuating coil of said solenoid.

19. The improvement in accordance with claim 17 wherein said circuitry is a switching circuit utilizing said actuating coil of said solenoid.

20. The improvement in accordance with claim 12 which additionally includes friction-reducing means to reduce the frictional drag between said core and said shiftable armature.

21. The improvement in accordance with claim 20 wherein said friction-reducing means comprises a micro-finish at the interface surfaces of said core and armature.

22. The improvement in accordance with claim 20 wherein said friction-reducing means comprises a sleeve of low-coefficient of friction material forming the core within which said shiftable armature isdisposed.

23. The improvement in accordance with claim 22 wherein said sleeve is formed of a self-lubricating synthetic material.

24. The improvement in accordance with claim 20 .wherein said friction-reducing means comprises spaced low-coefficient of material contact members forming a contracting surface between said armature and core of reduced frictional drag.

25. The improvement in accordance with claim 24 wherein said contact members are radially disposed plastic button members on the fore and aft ends of said armature.

26. The improvement in accordance with claim 24 wherein said contact members are continuous, annular, plastic members on the fore and aft end of said armature. 

1. The method of operating solenoids and solenoid-actuated mechanism under comparatively linear armature response conditions comprising: a. energizing the solenoid at relatively high current levels with respect to the mass of the armature of the solenoid and load forces thereon and maintaining said levels within selected limits; b. providing high magnetic leakage paths for said solenoid; and c. selectively varying said current within said selected limits to obtain substantially linear response of said armature correlative to said current.
 2. The method in accordance With claim 1 wherein said current levels are within the near-linear areas of the force versus stroke solenoid curves.
 3. The method in accordance with claim 1 wherein said high magnetic leakage paths are obtained through the provision of air gaps between said armature and the core of said solenoid.
 4. The method in accordance with claim 3 wherein additional high magnetic leakage paths are obtained by shading the diameter of said armature.
 5. The method in accordance with claim 4 which additionally includes varying the solenoid return spring characteristic in order to obtain said linear response.
 6. The method in accordance with claim 5 wherein the current supply to the coil of said solenoid is discontinued about 50 to 60 times per second.
 7. The method in accordance with claim 6 wherein the armature is operated under dither conditions, the current impulse cycle urging said armature toward the open position and said solenoid return spring urging said armature toward the closing position.
 8. The method in accordance with claim 4 which additionally includes obtaining said linear armature response under low mechanical resistance conditions.
 9. The method in accordance with claim 8 wherein the coefficient of friction of the materials comprising said armature and said core are maintained at a minimum.
 10. The method in accordance with claim 8 wherein said low mechanical resistance conditions are obtained by providing point contacts between armature and core of low frictional drag characteristics.
 11. In a solenoid comprising a core; shiftable armature and actuating coil, the improvement which comprises means to establish high saturation losses during operation of said solenoid in order to obtain substantially linear response of said armature correlative to increase in current inputs to said coil.
 12. The improvement in accordance with claim 11 wherein said means comprises shaded areas on said armature.
 13. The improvement in accordance with claim 12 which additionally includes a return armature spring adapted to provide a near-linear resistive force to the movement of said armature upon actuation of said solenoid.
 14. The improvement in accordance with claim 13 wherein said shading to said armature comprises an annular groove formed in the end of said armature furthest from said core.
 15. The improvement in accordance with claim 14 which additionally includes the formation of a conical air gap between the core and armature end opposite said groove.
 16. The improvement in accordance with claim 15 which additionally includes circuitry to periodically energize said solenoid.
 17. The improvement in accordance with claim 16 wherein said circuitry is located within the housing of said solenoid.
 18. The improvement in accordance with claim 17 wherein said circuitry is an interrupter circuit utilizing said actuating coil of said solenoid.
 19. The improvement in accordance with claim 17 wherein said circuitry is a switching circuit utilizing said actuating coil of said solenoid.
 20. The improvement in accordance with claim 12 which additionally includes friction-reducing means to reduce the frictional drag between said core and said shiftable armature.
 21. The improvement in accordance with claim 20 wherein said friction-reducing means comprises a micro-finish at the interface surfaces of said core and armature.
 22. The improvement in accordance with claim 20 wherein said friction-reducing means comprises a sleeve of low-coefficient of friction material forming the core within which said shiftable armature is disposed.
 23. The improvement in accordance with claim 22 wherein said sleeve is formed of a self-lubricating synthetic material.
 24. The improvement in accordance with claim 20 wherein said friction-reducing means comprises spaced low-coefficient of material contact members forming a contracting surface between said armature and core of reduced frictional drag.
 25. The improvement in accordance wiTh claim 24 wherein said contact members are radially disposed plastic button members on the fore and aft ends of said armature.
 26. The improvement in accordance with claim 24 wherein said contact members are continuous, annular, plastic members on the fore and aft end of said armature. 